This invention relates to a rotational speed sensor for sensing the rotational speed of a vane compressor.
Vane compressors are widely employed as refrigerant compressors in air coolers for vehicles and other air conditioners, by virtue of their compact sizes. A typical vane compressor of this kind comprises a pump housing formed by a cam ring having an endless camming inner peripheral surface and opposite side blocks secured to opposite ends of the cam ring, and a rotor received within the pump housing and carrying vanes radially movably disposed for sliding contact with the endless camming inner peripheral surface of the cam ring. As the rotor rotates, pump working chambers are defined between adjacent ones of the revolving vanes, the inner wall faces of the pump housing, and the outer peripheral surface of the rotor, wherein pumping actions are performed for suction, compression and discharge of refrigerant. The vane compressor constructed as above is driven by an engine by means of coupling means such as an electromagnetic clutch.
In a vane compressor employing an electromagnetic clutch as such coupling means, the rotational speed of the rotor does not always exactly correspond to the rotational speed of the engine due to slippage of the electromagnetic clutch, making it impossible to detect with accuracy the rotational speed of the rotor from that of the engine. This makes it difficult to achieve accurate control of the compressor, that is, control of the air conditioner.
A vane compressor of the above type has not a few sliding rotating parts. As a consequence, when the compressor is operating at a very high rotational speed in a condition where a reduced amount of refrigerant with lubricating oil mixed therein is present within the compressor, some sliding rotating parts such as the rotor can become short of necessary oil films, resulting in seizure of the rotor, etc., and even in breakdown of the compressor.
Therefore, to reduce the possibility of a trouble of this kind to a minimum, it is necessary to detect the actual rotational speed of the compressor as accurately as possible. However, so far as the present inventor knows, almost no rotational speed sensor for vane compressors which fully satisfies this requirement has so far been proposed.
On the other hand, in the field of swash plate type compressors which are also widely used in air conditioners, a rotational speed sensor has been proposed by Japanese Provisional Patent Publication No. 56-64183 which comprises a sensor of the electromagnetic type arranged in the cylinder block at such a location as to encounter a predetermined portion of the outer peripheral surface of the swash plate each time the swash plate makes one rotation to sense the rotational speed of the swash plate, or which comprises a sensor of the same type arranged in a cylinder bore at such a location as to encounter a recess formed in the outer peripheral surface of a piston received in the cylinder bore to sense a reciprocating motion of the same piston driven by the swash plate and thereby sense the rotational speed of the swash plate. Another rotational speed sensor for swash plate type compressors has been proposed by Japanese Provisional Patent Publication No. 57-59170 which comprises a sensor of the electromagnetic type disposed opposite the outer periphery of the swash plate to encounter a magnetic material such as an iron piece embedded in the outer peripheral surface of the swash plate each time the swash plate makes one rotation to sense the rotational speed of the swash plate.
However, vane compressors are basically different in structure from swash plate compressors such that the above proposed rotational speed sensors cannot be directly applied to the former. Further, the sensor proposed by the former publication makes it an essential requisite that the swash plate or the piston should be formed of a magnetic material.
Among rotational speed sensors for sensing the rotational speed of a rotary shaft applicable to machinery in general, a rotational speed sensor has been proposed by Japanese Provisional Patent Publication No. 57-56759 which comprises a sensor of the electromagnetic type arranged in the housing of a machine and disposed opposite an end of a rotary shaft to encounter a channel diametrically centrally formed across the end face of the rotary shaft each time the latter makes one rotation to sense the rotational speed of the rotary shaft. However, to apply this proposed sensor to a vane compressor, the compressor has to be designed large in axial size, which fails to meet a recent demand for designing vane compressors of this kind compact in size.
Moreover, it is a recent tendency that in order to reduce the weight of vane compressors, main parts of a vane compressor such as the pump housing and the rotor are made of nonferrous metals or light alloys, e.g. an aluminum base alloy. Since such materials are nonmagnetic materials or low magnetic permeability materials, it is almost impossible to apply the aforementioned proposed rotational sensors to vane compressors in a manner of utilizing the magnetism of the rotor for sensing the rotational speed of same.
Besides, in view of the structure of a vane compressor adapted to compress refrigerant to a very high pressure, consideration should be given to the location of the sensor body of a rotational speed sensor applied to the compressor so as not for the sensor body to be exposed to the high pressure refrigerant, to ensure a long life of the sensor.